Experimental Study On Effect Of Lubricating Oil On The Oxidation Reactivity Of Diesel Particles

Lubricating oil is one of the main sources of diesel particulate emission and ash deposition in diesel particulate filter.Therefore,it is of great significance to study the influence of lubricating oil on the oxidation reactivity and oxidation process of diesel particulate matter(PM),to provide theoretical guidance for the optimization of lubricating oil formula,and to control the emission of diesel particulate matter.In this paper,the effects of different lubricating oil components,including mineral base oil,synthetic base oil,ashless dispersant,detergent,antiwear additive,additive package and organic molybdenum on the size distribution of primary particles,nanostructure,graphitization degree and surface functional groups(SFGs)of diesel particulate matter generated by a heavy-duty engine were studied.The characteristic temperature and apparent activation energy of diesel soot particles were studied and the correlation between oxidation reactivity and the physicochemical properties of soot particles was analyzed.In addition,the impact of lubricant-derived ash on the oxidation mode of soot particles was studied by analyzing the size and nanostructure of the primary soot particles during the oxidation process.The major work and achievements of this dissertation are listed as follows:When the base oils and ashless additive participate in combustion,the fringe length(La)of soot particles decreases,the fringe tortuosity(Tf)increases,the graphitization degree decreases,the relative concentration of aliphatic C-H groups increases,the oxidation characteristic temperatures decrease,and the apparent activation energy decreases,indicating that the soot oxidation activity increases.Metal additives participate in combustion not only lead to the changes of the physicochemical properties,but also increase the ash content of diesel soot particles.The oxidation characteristics of soot particles are affected by their physicochemical properties and ash content.The results show that soot with a smaller La,a larger Tf,a lower degree of graphitization and more aliphatic C-H group content can enhance the soot oxidation reactivity.According to the linear correlation coefficient,R2,obtained by linear regression,the apparent activation energy of soot particles oxidation has a great correlation with the degree of graphitization and the content of aliphatic C-H group,but a little correlation with the size of primary particles.In the early stage of oxidation,most of the soot particles show a "shell-core" structure.As the oxidation proceeds,more and more onion-like structure is observed.The diameter of the primary particles decreases,which means that soot particles are in the middle of the surface burning process.The continuous oxidation of soot leads to the rearrangement graphene layer,and its internal structure is more and more graphitized.At the conversion of 40%,some hollow soot particles were observed,which indicated that the internal burning starts.In the later stage of oxidation(ca.60%),more and more onion-like and capsule-like structures are observed.There are also many carbonization fragments with a high graphitization degree and a size significantly smaller than other primary particles.There are both internal and surface oxidation modes in the oxidation process of diesel soot particles.In the early stage of oxidation,the surface oxidation is dominant.With the oxidation process,the internal oxidation mode gradually increases.Under the same oxidation conversion,compared with pure diesel,the internal oxidation tendency of soot particles generated after base oil participating in combustion is higher than that of pure diesel.The metal ash derived from the additive acts as a catalyst and promotes the surface oxidation of soot particles.